Know your limit. Drive safe.

The most accurate mathematical method to calculate Blood Alcohol Concentration is using Watson's Total Body Water (TBW) regression equations. Unlike the basic Widmark formula which relies on generic constants, the Watson method estimates the exact volume of water in your body based on your age, sex, weight, and height. Since alcohol is water-soluble, it distributes itself in total body water, making the Watson equation the most precise formula for estimating blood alcohol percentage before chemical analysis (such as a breathalyzer or blood test).

The human body clears alcohol at a constant linear rate of approximately 0.015% BAC per hour. This rate is fixed because the liver can only process a set amount of ethanol per hour regardless of how much you have consumed. For example, if your BAC peaks at 0.08%, it will take about 5.3 hours of pure metabolic activity to eliminate the alcohol and reach a 0.00% sober baseline. Note that drinking water, coffee, or taking cold showers does not speed up alcohol elimination.

Body mass and biological sex significantly impact your alcohol level. A higher body weight provides a larger volume of water (Total Body Water) to dilute the consumed alcohol, resulting in a lower BAC. Biological females typically have a lower body water percentage and higher fat tissue ratio than males of equal weight. Because fat does not absorb alcohol, females experience a higher blood alcohol concentration from the same amount of drinks. Our blood alcohol calculator male and blood alcohol calculator female profiles account for these critical biological differences.

Legal driving limits vary by country: In the United States, the federal legal limit for DUI is 0.08% BAC for drivers over 21 (Utah enforces 0.05% BAC). In the United Kingdom, the limit is 0.08% BAC in England, Wales, and Northern Ireland, while Scotland enforces a stricter limit of 0.05% BAC. In Australia, the national legal driving limit is 0.05% BAC for full license holders, and a strict 0.00% limit applies to learner and provisional drivers.

For an average 180 lb (81 kg) male, drinking three standard beers (355ml at 5% ABV) in one hour will yield a BAC of approximately 0.06%, which is over the limit in Australia and Scotland but below the 0.08% limit in the US. However, for a 130 lb (59 kg) female, those same three beers will push her BAC to around 0.095%, which is over the legal driving limit in the US and UK. The safest choice is always not to drive after drinking.

The calculator estimates your blood alcohol content by combining your biological profile (using Watson's formula) with the timestamp and volume of each drink. It models the absorption phase (which typically peaks within 30 to 90 minutes after drinking) and then applies a linear elimination rate of 0.015% per hour to project your estimated BAC over a continuous timeline.

Blood alcohol levels correspond to varying clinical stages of impairment. At 0.02% to 0.04% BAC, individuals typically experience mild relaxation and subtle mood elevation. By 0.05% to 0.07%, minor coordination issues, impaired judgment, and slower reaction times begin. At 0.08% BAC and above, cognitive function, depth perception, and critical motor skills are significantly degraded. While estimates provide a useful baseline, actual physical impairment varies based on individual tolerance, genetics, and stomach contents.

Your BAC changes dynamically based on the rate of absorption and elimination. After you finish a drink, the alcohol must be absorbed into your system, typically peaking in the blood within 30 to 90 minutes. Once absorption is complete, the liver processes and eliminates the alcohol at a steady linear rate of roughly 0.015% BAC per hour. If you drink rapidly (such as three drinks in under an hour), your peak BAC will occur shortly after you stop, followed by a slow, linear descent over the subsequent hours.